Inosilicates - Inosilicates (Chain silicates) Common...

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Unformatted text preview: Inosilicates (Chain silicates) Common Fe/Mg-bearing silicates two common groups:- pyroxenes: single chain- amphiboles: double chain pyroxenes more common because of MORB amphiboles more common on continents because of weathering Pyroxene Group XYZ 2 O 6 Single chains extend along c axis Z/O ratio = 1/3 Z cations usually Si, occasionally Al Chains are stacked along a axis, alternating:- base faces base- apex faces apex X cations in M2 sites- between bases of tetrahedrons- distorted 6- and 8- fold coordination- depends stacking and the size of the cations Y cations in M1 sites- 6-fold coordination between apical oxygen I-beams- consist of two chains connected by Y cations- closeness of apical oxygen and 6-fold coordination make bonds strong I-beams held together by X cations in M2 site 1- coordination depends on how chains line up- 6-fold coordination gives orthorhombic symmetry- 8-fold coordination gives monoclinic symmetry Crystal shapes:- blocky prisms, nearly square- elongate along c axis cleavage controlled by I-beams:- cleavage typically between 87 and 93- viewed down the c axis Classification of pyroxenes Based on two linked things:- cations in M2 site- symmetry Most plot on ternary diagram, apices:- Wollastonite, Wo- Enstatite, En- Ferrosilite, Fe Three major groups- orthopyroxenes (opx) orthorhombic- low-Ca orthopyroxenes (opx) orthorhombic- Ca-rich clinopyroxenes (cpx) monoclinic Orthopyroxenes: Fe and Mg, little Ca- both M1 and M2 are octahedral (small)- larger Fe ion fits mostly in M2 site (larger) 2 Low-Ca orthopyroxene: more Ca, but no solid solution with clinopyroxene- mineral species is Pigeonite- Ca restricted to M2 sites, these still mostly Fe and Mg...
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This note was uploaded on 07/06/2011 for the course GLY 5245 taught by Professor Staff during the Spring '11 term at University of Florida.

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Inosilicates - Inosilicates (Chain silicates) Common...

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